Know yourself. Remember the Five Foundations: don't go into the interview thinking that your goal is to prove to the interviewer that you're worthy. Your goal is to get to the know the school and let the school get to know you. In order to do this, you have to go through the work of getting to know yourself. What are your strongest qualities and attributes? Where have you messed up, and what have you learned from it? What are you hoping to offer the school other than your money and time in a classroom? In what types of situations do you excel? What types of situations are challenges for you? How do you work through and overcome challenges? What are you hoping to get from college that you haven't even told anyone about yet?
Remember that the school already has your transcript and application, or they will soon. So don't spend much time telling them things they already know about you. The interview is one of your best opportunities to prove that you're a person, not a resume. So let your human qualities show.
Know the school. Your interviewer doesn't need you to tell them how great the school is. They already know that. Flattery isn't going to get you far. What you need to know, and what you want to be able to explain, is what specifically about the school makes you think it's a good match for you. Maybe you've already done a lot of research and you know some very specific things about the school that attract you to it. Maybe you haven't. But when you're asked "what makes you interested in our university?" you want to have a better answer than "I hear it's good," or "My friend is applying here so I thought I would too." Before each interview, go through the school's web site and brochures to remind yourself of specific things that draw you to the school. You might feel like they're trivial or silly, but if those factors are important to you then everyone should know.
Your mindset is critical: in any interview, you're not trying to get into college. You're trying to decide if this specific college is a good match for you...and if this specific college thinks you're a good match for it.
As much as possible, find out about your interviewer. Will you be talking to a counselor from the Admissions department? A professor? A dedicated alumnus? A group of people? Look up the biography for each person you know you're going to talk to. This will make you more comfortable when you get into the room, because they'll seem more familiar. This will give you a sense of what they might be looking for. This will give you some ideas for questions you might have. If you're talking to an alum or other volunteer, ask them what makes them so passionate about the school they're willing to donate their time for it.
Practice. Have practice interview sessions with family. Join a group of friends who are all going through the admissions process and practice responses to typical questions. Just like actors with dress rehearsals, athletes with scrimmages, or lawyers with mock trials, you don't want your first interview experience to be a real interview.
If you're going to prepare memorized answers, don't memorize more than 60 seconds--total. People can tell when you're talking to them and when you're reciting a speech, and real talk is better. even if it's less polished.
Pay attention to your words, but also your body. Don't spend too much time trying to completely reprogram your body language. Instead, choose three specific stances or movements you want to be mindful of and practice them over and over. When you are going in for the interview, repeat to yourself "I will remember to smile, make eye contact, and sit up straight," or "I will remember to keep my feet flat on the floor, keep my hands open, and pause two seconds before answering a question." Whatever your big three are, focus on them and practice them. A lot. These will be useful to you in many situations, not just college interviews.
Dress YOUR best. If you're dressed up and comfortable, you exude confidence. If you're dressed up and uncomfortable, you look like a little kid wearing scratchy clothes. So wear the best clothes that you normally wear. If you go to a school that requires you to wear a jacket or skirt every day, you're going to be comfortable in that, and should stick with that. If you normally wear jeans and a t-shirt, then wearing pressed pants and a clean shirt might be the better route for you if you'll look out of place even more dressed up.
Strangely, the main point of dressing well is for people to not notice your clothes. You want your interviewer paying attention to your words and face, not your clothes. Dress too casually or wear dirty clothes, and your interviewer wonders why you don't take them more seriously. Dress so fancy you're twitching or look out of place, and the interviewer wonders why you lack confidence. So know yourself, know how you want to present yourself, and then give the best version of that. Don't wear anything that stands out too much.
Thank the interviewer. It's not just that you hope the interviewer will give you something by recommending you to be accepted. It's also that the interviewer has already given you something by giving you their time and giving you an opportunity to explain yourself. If nothing else, the interviewer has given you good real-world practice for other interviews down the line. Thank your interviewer for their time, in person, while you're in the room. Also send a follow-up thank you. Some people will insist that you send a hand-written note, but I think an email is fine. A hand-written card is just as likely to read "Her mother made her do this" as "She must really be interested in this school." The way to make your thank you stand out is to include something specific about your interview that the person will remember. Don't just send a vague form note, and definitely don't copy and paste a thank you that you've already sent.
There's already a whole lot of interview advice out there, and don't be afraid to spend a little time looking at it. But the most productive and efficient thing you can do, both in the short term and long run, is spend time getting to know yourself and practice explaining yourself to others. That will make you the type of interesting person that other interesting people want to be around.